Process of making cold reduced steel

ABSTRACT

A making process of cold reduced steel, which is specially produced by continuous annealing line, characterized by making ingot that carbon is from 0.03% to 0.06% and oxygen is from 0.05% to 0.10%, decarburizing the hot strip at temperature of less than 780*C, and finally annealing said strip at temperature of more than recrystallization temperature to A3 point.

United States Patent [1 1 Matusdo et al.

[ PROCESS OF MAKING COLD REDUCED STEEL [75] Inventors: Kazuo Matusdo; Takayoshi Shimomura; Kooichi Oosawa, all of Fukuyama, Japan [73] Assignee: Nippon Kokan Kabushiki Kaisha Chiyoda-ku, Tokyo, Japan 22 Filed: Jan. 25, 1971 21 Appl. No.: 109,446

[30] Foreign Application Priority Data Feb, 2, 1970 Japan 45-8451 [52] U.S. Cl. l48/12.l, 148/12 C [51] Int. Cl C2ld 9/48 [58] Field of Search 148/12, 12.1

[56] References Cited UNITED STATES PATENTS 3,178,318 4/1965 Shimizu et a1. 148/12 9 REGION OF UNIFORM GRA N DECARBURIZATION TEMPERATURE [451 Feb. 11,1975

3,215,567 11/1965 Yoshida 148/12 3,239,390 3/1966 Matsukura l48/12.1 3,248,270 4/1966 Laidman et a1 148/12 3,264,144 8/1966 Frazier et a1 148/12 Primary Examiner-W. Stallard Attorney, Agent, or Firm-U1Ie C. Linton [57] ABSTRACT 4 Claims, 1 Drawing Figure REGION OF CARSENING GRAIN 700 0 Q o W 000 w o X am 0 e o o 0 W006 w 8 X 61 0.07 0.00 004 000 0.02

O UNIFORM GRAIN 0 MIDDLING COARSENING GRAIN O COARSENING GRAIN V LAYER OF CENTER PORTION NOT DECARBURIZED PROCESS OF MAKING COLD REDUCED STEEL This invention relates to an improved process of making cold reduced steel having good press formability which is specially possible to be produced in large scale by continuous annealing line.

It is well known that there are many papers or patents in the field of deep drawable steel making, which is by means of adjusting chemical composition in steel, setting up rolling condition of the steel, and/or controlling heat treatment of the steel. These processes are commonly employed to make good drawable steel and very effective in making said steel. It is, however, a fact that there is a limit of itself in said drawability of ordinary rimmed steel, even though the known decarburization annealing process is added to the final stage. That is, the above decarburization annealing process have a tendency to make crystal grain coarse and result in bringing about known orange peel on surface of the steel in press forming stage. We inventers experiments show that steel should be annealed at temperature of about 700C in order to avoid such phenomenon. It will be impossible to improve said drawability of the steel.

This invention is developed to improve the above fault and to make the effect of said decarburizing process demonstrate fully. The features of this invention lie in decarburizing hot strip before cold reducing of the strip. How to treat the decarburized hot strip is sufficient to depend on ordinary steps, which is coldreduced, box-annealed or passed through continuous annealing line and then temper-rolled. Press formability of said strip treated with the above steps in far better than that of the common process.

Thus, an object of this invention is to provide and improved process of making cold reduced steel having for more excellent press forwability than that of the steel produced by conventional process.

Another object of this invention is to provide an improved process of making cold reduced steel being capable to demonstrate good drawability even in the case of being treated with common continuous annealing line.

Other objects and advantages will be apparent with the following detailed description and by the accompanying drawing in which:

The accompanying drawing shows interrelation and amount of oxygen in rimmed steel, which have direct effects upon ferrite grain.

In this invention, low carbon rimmed steel ingot should be made within the range of 0.03% to 0.06% carbon and 0.05% to 0.10% oxygen in steel making furnace. The hot strip, which is rolled from the above ingot, is decarburized at less than 780C temperature and then is cold-reduced in the ordinary manner. Such a cold-reduced strip is finally annealed at a temperature of less than A point to more than recrystallization temperature of the steel and then is temper-rolled.

ln the above process, the reason that said carbon content is limited to the range of 0.03% to 0.06% is as follows: less than 0.03% carbon is impossible to be made with the ordinary steel making process, and more than 0.06% carbon is difficult to be effectively decarburized. The reason that said oxygen content is limited to the range of 0.05% to 0.10% lies in that less than 0.05% oxygen causes said ferrite grain to coarsen, and more than 0.10% oxygen gives a great damage to cleanliness of the steel. The interrelation between the above oxygen content and decarburization temperature is the most improtant factor in this invention. The accompanying drawing shows the results of the above interrelation, which is obtained by many experiments.

Referring now to said drawing, it is possible to be well understood that the scope that uniform grain is possible to be obtained is the region enclosed by oxygen content of more than 0.05% and decarburization temperature of less than 780C. However, the region of more than 0.06% oxygen and less than 750C should be recommended as the most preferable scope. The scope of less than 0.05% oxygen and more than 770C decarburization temperature is scarcely employed, because it is confirmed as the region of coarsening grain. This is the reason that decarburization temperature is limited to less than 780C. Such a decarburizing annealing is carried out with common open coil annealing furnace.

The strip after cold reducing is annealed at a temper ature of more than recrystallization temperature to less than A point. Such a final annealing can be put into practice with either box type annealing furnace of continuous annealing furnace. It is, in the case of the latter, confirmed that there is shown many advantages in comparison with the common continuous annealing process. Firstly, the mechanical properties of this invention steel is far better than that of the steel produced by common continuous annealing process, as mentioned later. Secondly, while decarburization annealing in this invention is carried out for hot rolled strip, solution carbon in ferrite becomes very low, consequently aging resistibility of the steel is possible to be improved. Thirdly, in this invention, the annealing temperature near A point is possible to be employed as mentioned later. Even though such a high temperature, there is no coarsening of recrystallized grain. This is a large advantage and a great improvement of mechanical properties is obtained. It is needless to say that the same effect as mentioned above will be obtained in the case of said box type annealing.

Actual example of this invention process are as fol lows:

EXAMPLE 1 Steel making furnace: LD convertor Chemical composition: as in Table I Hot rolling condition after slabbing:

finishing temperature: 850C coiling temperature: 595C thickness of strip: 2.3mm

Decarburizing condition after pickling:

employed annealing furnace: atmosphere:

open coil furnace wet hydrogen soaking temperature: 730C X 10hr. carbon in steel: 0.003%

Cold reducing condition:

reducing ratio: 739% thickness of strip: 0.6mm

Final annealing condition:

employed furnace: ordinary continuous annealing furnace 700C X l min.

soaking temperature: Temper rolling ratio:

The mechanical properties of the above mentioned steel is shownin Table II.

Table ll Yield point(Kg/mm Elongation of yield point(7z Tensile strength(Kg/mm Total elongation (70): Lanford value r:

Aging index (Kg/mm"): Hardness HR'- BOT:

EXAMPLE ll Producing requirments are the same as those of Example l, except soaking temperature of the continuous annealing.

Soaking temperature: 800C X1 min. The mechanical properties of the above lows:

steel is as fol- Table III Yield point (Kg/mm Elongation of yield point (71): Tensile strength (Kg/mm): Total elongation (71): Lanford value r:

Aging index (Kg/mm): Hardness HR 30T:

A steel compared with Example 1 and ll was produced by the ordinary continuous annealing process. That is, the making conditions are the same as those of said Example, except the annealing condition. Annealing condition: y decarburization: not employed doaking temperature of continuous annealing process: 700C X l min The mechanical properties of the steel is as follows:

Table IV Yield point (Kg/mm): Elongation of yield point (71): Tensile strength (Kg/mm Total elongation (7t Lanford value r:-

Aging indcx (Kg/mm): Hardness HR 30T:

ALA f r r'r-Pn O T-MAOC KA Hot rolling condition after slabbing:

Table V-Continued finishing temperature: coiling temperature: thickness of strip:

Decarburizing condition after pickling:

employed furnace: atmosphere:

soaking temperature: carbon in steel:

Cold reducing condition: reducing ratio: thickness of strip:

Final annealing condition:

employed furnace:

- atmosphere:

soaking temperature: Temper rolling ratio:

' open coil annealing furnace wet hydrogen ordinary ti ht coil annealing urnace HNX gas The mechanical properties of the abobe steel are shown in Table VI.

Table VI Yield point (Kg/mm):

Elongation of yeild point Tensile strength (Kglmm Total elongation (70): Lanford value r: Aging index (Kg/mrfl Hardness HR 30T:

LIIN

EXAMPLE lV Producing requirements are the same as those of Example lll, except soaking temperature of the box type annealing,

Soaking temperature: 800C 5hr The mechanical properties of the above steel are as follows:

Table VII Yield point (Kg/mm Elongation of yeild point (70): Tensile strength (Kg/mm):

Total elongation (70)1 Lanford value r:

Aging index (Kg/mm): Hardness HR -30T:

A steel compared with Examplex lll and IV was produced by the ordinary box type annealing process. That is, the making conditions are the same as those of said Example, except the annealing condition:

Annealing condition:

decarburization: not employed soaking temperature of box type annealing process: 700C X 5hr The mechanical properties of the steel are as follows:-

Table Vlll Yield point (Kg/mm Elongation of yield point (70): Tensile strength (Kg/mm):

Total elongation /1 Lanford value r:

Aging index (Kg/mm Hardness HR SOT:

According to the above Table Vlll, it will be understood that all of the mechanical properties are far lower than those of Example VI and VII.

Thus, it is understood that the process of this invention is making splendid accomplishments. We must not take that it was itself necessary. That is, the first reason lies in the final annealing for the steel that carbon content in steel is very low. The second reason is that said first reason causes growth of recrystallizing grain and make texture of (111) direction greater in occupying rate. Accordingly, press formability of this invention steel is far higher than that of usual steel, even though its grain size is the same as that of another steel.

What is claimed is:

1. An improved process of making cold reduced steel consisting in producing low carbon rimmed steel of 0.03 to 0.06% C and 0.05 to 0.10% O, decarburizing said steel at less than 780C, cold-reducing said steel,

ing temperature is less than 750C. 

1. AN IMPROVED PROCESS OF MAKING COLD REDUCED STEEL CONSISTING IN PRODUCING LOW CARBON RIMMED STEEL OF 0.03 TO 0.06% C AND 0.05 TO 0.10% 0, DECARBURIZING SAID STEEL AT LEAST THAN 780*C, COLD-REDUCING SAID STEEL, FINALLY ANNEALING SAID COLDREDUCED STEEL AT LEAST THAN A3 POINT TO MORE THAN RECRYSTALLIZATION TEMPERATURE AND THEN TEMPER-ROLLING SAID STEEL.
 2. An improved process of making cold reduced steel set forth in claim 1 wherein the final annealing is continuous annealing.
 3. An improved process of making cold reduced steel set forth in claim 1 wherein the final annealing is box type annealing.
 4. An improved process of making cold reduced steel set forth in claim 1 wherein the oxygen content is within the range of 0.06 to 0.10% and the decarburizing temperature is less than 750*C. 